Add comprehensive GitHub Copilot instructions for .NET Performance repository

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed, validated instructions for working effectively in the .NET Performance repository.

Key Features

The instructions file provides:

• Firm directive to always reference these instructions first before falling back to search or bash commands
• Complete bootstrap process with exact .NET SDK version requirements (10.0.100-preview.7.25322.101)
• Validated command sequences with precise timing expectations and timeout requirements
• Network limitation awareness with workarounds for sandboxed environments that cannot access Microsoft's internal package feeds
• Imperative tone throughout with specific, copy-pasteable command examples

Validated Components

All commands and timings have been thoroughly tested:

• .NET SDK installation: ~15 seconds using python3 scripts/dotnet.py install
• Scenario test workflows: 5-15 minutes with proper pre/test/post command sequences
• Python environment setup: Including gitpython dependencies and --allow-non-admin-execution flags
• Markdownlint validation: ~20 seconds for full repository, ~0.6 seconds for docs only
• Build system limitations: Documents expected 3-5 minute failures due to Microsoft.DotNet.Arcade.Sdk dependencies

Critical Timing Warnings

The instructions include explicit "NEVER CANCEL" warnings with specific timeout recommendations:

• Build operations: 60+ minutes timeout
• Scenario tests: 30+ minutes timeout
• npm installs: 60+ minutes timeout

Environment Setup

Provides complete environment variable configuration that works across different working directories:

export PATH="$PWD/tools/dotnet:$PATH"
export DOTNET_ROOT="$PWD/tools/dotnet"
export DOTNET_CLI_TELEMETRY_OPTOUT='1'
export DOTNET_MULTILEVEL_LOOKUP='0'
export UseSharedCompilation='false'

Validation Scenarios

Includes three essential validation workflows for testing changes:

1. Scenario infrastructure validation
2. Python script environment confirmation
3. Documentation linting

The instructions acknowledge that this is a specialized performance benchmarking repository where timing is critical, network dependencies are common, and build failures in sandboxed environments are expected but workarounds exist.

Fixes #4906.

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